The invention relates to a paving screed for a road paver, the paving screed comprising
a base screed and at least one extension screed which is provided at the base screed offset in a working travelling direction and which is extendable and retractable relative to the base screed in linear sliding direction,
a base guiding structure for the extension screed, the base guiding structure being pivotable in a pivot suspension of the base screed,
a guiding sub-structure being guided in the base guiding structure and at least a first actuator for sliding the guiding sub-structure over first stroke relative to the base guiding structure,
an extension guiding structure guided in the guiding sub-structure, and at least a second actuator for sliding the extension guiding structure over a second stroke relative to the guiding sub-structure,
a sole plate frame structure mounted to the extension guiding structure and having an extension screed sole plate, and
vertical guidances and elevation adjustment assemblies extending substantially vertical in relation to the sliding direction, and provided between the sole plate frame structure and the extension guiding structure, for adjusting the elevation of the sole plate frame structure parallel to itself and relative to the base guiding structure.
Front-mount paving screeds having extension screeds at the front side of the base screed for being extended and retracted sidewardly in relation to the base screed to vary the working width (e.g. US 2007/0258769 A1) are used predominantly in Northern America because front-mount extension screeds provided at the front side of the base screed have advantages over rear-mounted paving screeds having the extension screeds at the rear side of the base screed, in particular when working at high working speed in working travelling direction while varying the working width and/or while forming sideways connections of drives or crossroads. A further frequently occurring requirement is to form roadbeds with a sidewardly inclined slope and/or a berm. Both, the slope and/or berm can be produced with relatively high working travelling speed if the extension screeds are mounted to the front side of the base screed. A slope e.g. has the purpose to direct rainfall water sidewardly, while a berm is a counter-slope ascending from the slope or from the road surface and e.g. serves to direct sewage water along the roadside. When forming a slope the width of the road surface (a flat road surface or a road surface having a crown profile) should remain unchanged when the working width is varied, while the width of the slope may vary.
In the front-mount paving screed known from US 2007/0258769 A1 with extension screeds mounted to the front side of the base screed, the base guiding structure is pivoted relative to the base screed in the base screed by at least one vertical actuator. The guiding sub-structure is slidably guided in the base guiding structure. The extension guiding structure, finally, is slidably guided at the guiding sub-structure. The base guiding structure, the guiding sub-structure and the extension guiding structure define a common linear sliding direction of the extension screed. The sole plate frame structure is fixedly mounted to the extension guiding structure. Alternatively the sole plate frame structure may be inclined by an auxiliary actuator in lateral direction relative to the extension guiding structure. The vertical actuator in the base screed is also used to carry out elevation adjustments of the extension screed in relation to the base screed. However, that actuator has to support all weight forces of the extension screed as well as all working forces resulting from the drag resistance of the paving material at the extension screed. In particular when the extension screed is fully extended such forces may become relatively high, which means that adjustments of the elevation of the extension screed when needed during the working process may be hindered. The double function (adjustment of the elevation and pivoting the extension screed relative to the base screed) of the vertical actuator of a pair of vertical actuators, furthermore, result in undue high local loads between the base guiding structure, the base screed and the actuator or the actuators.
In the front-mount paving screed known from U.S. Pat. No. 4,379,653 (FIGS. 17 to 24) an incorporated slope hinge of the base guiding structure of the extension screed is provided in the base screed. The base guiding structure of the extension screed can be pivoted in the slope hinge by means of a turnbuckle supported at the base screed in order to set the angle for a slope to be formed. A scissor lever mechanism elevation adjustment assembly is arranged between the slope hinge and the base guiding structure to vary the elevation of the sole plate of the extension screed in relation to the sole plate of the base screed. The extension guiding structure is telescopically slidable in the base guiding structure. As for variations of the working width, only a limited sort stroke of the extension guiding structure in the base guiding structure can be used, the working width cannot be varied to a measure corresponding with the twofold width of the base screed. In order to avoid that paving material lying on the planum is clamped in-between the extension screeds when the working width is markedly reduced, plough structures are formed at the front side inner ends of the extension screed. The extension screed sole plate is mounted at a sole plate frame structure fixed to the extension guiding structure.
In the front-mount paving screed known from U.S. Pat. No. 4,818,140 having extension screeds at the front side of the base screed the base guiding structure consisting of rails is pivoted by a jack screw at the base screed. In addition, the elevation of the base guiding structure can be adjusted vertically relative to the base screed by two spaced apart screw jacks. The extension screed sole plate is mounted via the sole plate frame structure to the shell-shaped extension guiding structure which in turn can be displaced in the base guiding structure. The extension screed sole plate is divided at a berm pivot region into two parts. In order to form a berm one part can be pivoted upwardly relative to an end gate by an actuator. The actuator is supported at the extension guiding structure. The working width of the paving screed cannot be varied to a measure corresponding to the twofold width of the base screed.
In the paving screed known from US 2002/01062443 A and US 2002/0106242 A1 the extension screed sole plate is sub-divided in a berm hinge into a first inner section and a second outer berm section. The entire extension screed sole plate can be tilted by a mechanism relative to the sole plate frame structure about a lateral axis extending parallel to the sliding direction, in order to change the attack angle of the extension screed sole plate individually.